1. Field of the Invention
The present invention relates generally to electro-optical inspection systems, and more particularly to an automated photomask inspection apparatus for detecting defects on optical masks and reticles and the like.
2. Brief Description of the Prior Art
Integrated circuits are made by photolithographic processes which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. A high production yield is contingent on having defectless masks and reticles. Since it is inevitable that defects will occur in the mask, these defects have to be found and repaired prior to using the mask.
Automated mask inspection systems have existed for over 15 years. The earliest such system, the Bell Telephone Laboratories AXMIS system (John Bruning et al., xe2x80x9cAn Automated Mask Inspection Systemxe2x80x94AMISxe2x80x9d, IEEE Transactions on Electron Devices, Vol. ED-22, No. 7 July 1971, pp 487 to 495), used a laser that scanned the mask. Subsequent systems used a linear sensor to inspect an image projected by the mask, such as described by Levy et al. (U.S. Pat. No. 4,247,203, xe2x80x9cAutomatic Photomask Inspection System and Apparatusxe2x80x9d) who teach die-to-die inspection, i.e., inspection of two adjacent dice by comparing them to each other. Alternately, Danielson et al. (U.S. Pat. No. 4,926,489, xe2x80x9cReticle Inspection Systemxe2x80x9d) teach die-to-database inspection, i.e. inspection of the reticle by comparison to the database from which the reticle was made.
As the complexity of the integrated circuits has increased, so has the demand on the inspection process. Both the need for resolving smaller defects and for inspecting larger areas have resulted in much greater speed requirements, in terms of number of picture elements per second processed. The increased demands have given rise to improvements described in a number of subsequently issued patents, such as U.S. Pat. No. 4,247,203, entitled xe2x80x9cAutomatic Photomask Inspection System and Apparatusxe2x80x9d, Levy et al., issued Jan. 27, 1981; U.S. Pat. No. 4,579,455, entitled xe2x80x9cPhotomask Inspection Apparatus and Method with Improved Defect Detectionxe2x80x9d, Levy et al., issued Apr. 1, 1986; U.S. Pat. No. 4,633,504, entitled xe2x80x9cAutomatic Photomask Inspection System Having Image Enhancement Meansxe2x80x9d, Mark J. Wihl, issued Dec. 30, 1986; and U.S. Pat. No. 4,805,123, entitled xe2x80x9cAutomatic Photomask Inspection and Reticle Inspection Method and Apparatus Including Improved Defect Detector and Alignment Subsystemxe2x80x9d, Specht et al., issued Feb. 14, 1989. Also of relevance is some prior art in the wafer inspection area, such as U.S. Pat. No. 4,644,172, entitled xe2x80x9cElectronic Control of an Automatic Wafer Inspection Systemxe2x80x9d, Sandland et al., issued Feb. 17, 1987.
Another force driving the development of improved inspection techniques is the emergence of phase shift mask technology. With this technology it will be possible to print finer linewidths, down to 0.25 micrometers or less. This technology is described by Burn J. Lin, xe2x80x9cPhase-Shifting and Other Challenges in Optical Mask Technologyxe2x80x9d, Proceedings of the 10th Annual Symposium on Microlithography, SPIE,xe2x80x94the International Society of Optical Engineering, Vol. 1496, pages 54 to 79.
The above improvements teach the automatic detection of defects on conventional optical masks and reticles. In all of these systems, conventional lighting is used and the images are captured by linear array sensors. These two system choices limit the signal-to-noise ratio and hence the speed of inspection.
An important object of present invention is to provide a novel defect detection apparatus which can use both transmitted and reflected light to inspect a substrate.
Another object of the present invention to provide a device of the type described in which surface elevations above a reference elevation are optically determined using inter-ferrometric principals and used as indicators of defects.
Another object of the present invention is to provide a device of the type described which uses the same optical system to detect defects and measure line widths.
Briefly, a preferred embodiment of the present invention includes an XY state (12) for transporting a substrate (14) under test in a serpentine path in an XY plane, an optical system (16) comprising a laser (30), a transmission light detector (34), a reflected light detector (36), optical elements defining reference beam paths and illuminating beam paths between the laser, the substrate and the detectors and an acousto-optical beam scanner (40, 42) for reciprocatingly scanning the illuminating and reference beams relative to the substrate surface, and an electronic control, analysis and display system for controlling the operation of the stage and optical system and for interpreting and storing the signals output by the detectors. The apparatus can operate in a die-to-die comparison mode or a die-to-database mode.
One advantage of the present invention is that it uses a laser light source and hence has a much higher brightness to scan the mask. It differs from the AMIS system described by Bruning et al. in that it employs an electro-optical deflection method instead of a mechanical system. Obviously the electro-optical method is faster and more flexible than a mechanical device. However, even conventional electro-optical deflections do not have sufficient speed to meet system requirements. In the present invention the speed is further enhanced by the use of a deflection apparatus previously described for laser beam recording by U.S. Pat. No. 3,851,951 to Jason H. Eveleth, entitled xe2x80x9cHigh Resolution Laser Beam Recorder with Self-Focusing Acousto-Optic Scannerxe2x80x9d, issued Dec. 3, 1974.
Another advantage is the use of a stage that has only two degrees of freedom. Prior art also incorporated a rotational capability at a considerable cost and complexity. In the present invention the effective direction of scanning is controlled by driving both axes of the stage simultaneously.
Another significant departure from previous art is the ability of the present system to simultaneously detect defects with both transmitted and reflected light. This capability is significant because the additional information can be helpful in determining the nature of the defect and thereby permits the automatic classification of defects.
Yet another advantage of the present invention is its ability to inspect phase shift masks. It is anticipated that phase shift mask technology will be used in the 1990""s to achieve linewidths of 0.10 micrometers. In the present invention the phase shift material can be measured at all points on a mask area at the normal scanning speed of the system.
Also advantageous is the ability of the present system to perform linewidth measurement on the mask. This is a significant advantage because heretofore two different types of instruments were employed to do both defect detection and linewidth measurement. The ability to use a single instrument results in a saving of time and, possibly more important, in less handling of the mask, which in turn is significant in contamination control.
A novel feature of the present invention is the autofocusing method employed. Previous mask inspection systems used autofocus systems that were affected by the pattern on the mask. The present invention functions independently of the pattern.
A significant innovation of the present system is also the two-axis preloading of the stage air bearings. Exceptional stiffness is achieved by this angular loading method.
Also new is the method of correcting for variations of light intensity. In the prior art the spatial non-uniformity of the illumination was determined before an inspection but no provisions existed for compensating for changing non-uniformity during inspection or, more likely, variations of the absolute level of intensity during the inspection. In the present invention the intensity is constantly monitored and immediately compensated in real time. Hence, variations of the primary light source with time do not affect the accuracy of the inspection process.
Yet another new capability of the present invention is to inspect the mask at substantially the same wave length as used for wafer printing (exposure) through the mask. With advances in technology, increasingly shorter wavelengths are used for printing. Because the appearance of defects changes depending on the wavelength of the illumination, it is important to employ approximately the same wavelength light source for both inspection and printing.
These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed disclosure of the preferred embodiments illustrated in the several figures of the drawing.